Conventionally, an intelligent modem system incorporated with a microprocessor unit was able to operate selectively in any of the three modes, the command mode, the transparent mode or the escape mode.
In the command mode, the modem LSI (large-scale integrated circuit) of the system is separated from the public communication line as well as from the mother computer so that various functions of the modem LSI, such as the on-off of the echoing of transmitted data, and the activation of a speaker, setting up of the limit of the time interval of the absence of a carrier before the modem system is disconnected from the public communication line, etc., may be set up as desired according to the command from the mother computer.
In the transparent mode, the modem LSI is connected to the public communication line as well as to the mother computer in order to perform data communication in normal fashion.
In the escape mode, the modem LSI is connected to the public communication line but is disconnected from the mother computer so that the function of the modem LSI may be switched from one kind to another according to the command from the mother computer without disconnecting the modem LSI from the public communication line. Therefore, according to this mode, a desired change in the function of the modem LSI can be accomplished while the data communication is being carried out in the transparent mode without disconnecting the system from the public communication line as opposed to the command mode.
Conventionally, the switch-over from the transparent mode to the escape mode has been accomplished by transmitting a certain character string from the mother computer to the modem system. However, according to this method, since the character string must be entered from the keyboard of the mother computer every time the operation mode is desired to be changed, not only the operation itself is substantially cumbersome but also this character string cannot be a part of the data to be communicated. Furthermore, the modem system must always monitor the communicated data if it contains this character string or not, and the system program becomes complicated accordingly.